Transpiration in plants is strongly driven by environmental conditions2. It is therefore difficult to compare the transpiration rate per plant among different species unless these plants are exposed to the same environment. This does not apply here, as Enquist et al. derived their transpiration rates from a re-analysis of data from the literature. The confounding effect is negligible at the scale of individual plants, given the wide range of rates and dimensions reviewed. But when scaling up to the whole canopy, environmental driving variables such as radiation, water availability and site fertility predominate and tend to obscure any effects of plant size on the function of stands of species. This is clear from a comparison of maximum conductances and assimilation rates among biomes across the world3.
The transpiration rates that have been scaled up to the level of populations by Enquist et al.1 (see their Fig. 4) seem surprisingly high: rates of about 100 l m-2 per day (that is, millimetres per day) far exceed the maximum values of 3-12 mm per day derived from a global comparison of plant canopies all over the world4. Another meta-analysis of evapotranspiration in coniferous forests and grasslands gave maximum values of 6-7 mm per day (ref. 5).
What is most important, however, is that the conclusions drawn by Enquist et al. from their upscaling to whole populations are misleading. It is debatable whether “total energy use or productivity of plants in ecosystems is⃛ invariant with respect to body size”: stand chronosequences in forest tree species6 indicate that, after canopy closure at the polestage, the leaf-area index tends to decline (as a result of self-thinning, among other processes). In mature canopies, this has a marginal effect on the interception of radiant energy and gross primary production7. Net assimilation and above-ground allocation, however, are further reduced by increasing respiratory costs, nutrient immobilization in soil litters and hydraulic constraints7,10, which are all a direct result of increasing body size, contributing to the well-known decline in forest growth with tree dimensions and age6,8.
Even the conservative nature of forest evapotranspiration on the regional scale11 seems to be true more at the community than at the population level, resulting from interaction among overstorey and understorey processes3. On the contrary, there are considerable changes in transpiration with stand development12. The results of Enquist et al. do not seem to alter this picture.
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Variation in root:shoot ratios induced the differences between above and belowground mass?density relationships along an aridity gradient
Acta Oecologica (2010)
Journal of Ecology (2006)